The present invention relates to read-write data storage devices and particularly to a method and system for defining write inhibit constraints and settle delay based on head geometry.
Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks that extend circumferentially around the disk. Each data track stores data in the form of a series of variations in disk surface magnetic orientation recorded laterally in the tracks. A magnetic read/write head suspended over the disk surface transduces the variations in magnetic orientation and produces a readback signal. The magnetic read/write head detects the variations in magnetic orientation and generates the readback signal as the disk rotates relative to the read/write head at thousands of revolutions per minute.
Conventionally, the read/write head is mounted on an actuator arm that is moved by a servo. A disk drive servo control system controls movement of the arm radially across the disk to move the read/write head from track to track in a seek. The servo control system produces a position error signal (PES) which provides fine position control for the disk. servo arm. Once the head is over a selected track, the servo control system maintains the head in a path over the track in a track following operation. Preferably, the head should be centered over the centerline of the magnetic transitions of the data track. If the head is not centered properly, the head may transduce transitions from the wrong track and create memory or data storage failure. What all systems ultimately try to avoid is the non-recoverable loss of data (the xe2x80x9chard errorxe2x80x9d). The rate at which the hard error occurs (hard error rate) should be as close to zero as possible. Consequently, the design parameters are selected conservatively.
One of the most crucial parameters of conventional read/write head design relates to the write inhibit windows. The write inhibit window is a measurement of how far off track the servo will permit the head to drift before interrupting the write operation. Somewhat farther from track center than this is the threshold at which adjacent track damage occurs. The write inhibit window is selected so that for all heads, there is sufficient margin between the window and the threshold of damage. Accordingly, the write inhibit window must be selected conservatively. However, the conservative nature of this choice can reduce performance on the many heads that could well operate within a wider window without the risk of adjacent track damage.
Combined with the risk of adjacent track damage is the risk of reading xe2x80x9cold informationxe2x80x9d, or data that should have been overwritten by a prior operation. If the head is off track while writing, not only may it cause damage to an adjacent track, but the new information may not fully replace the old information. In this case, there is a risk that if the head moves off track in the other direction on a subsequent operation, it may recover the old data, instead of the new, correct information. This can also drive the write inhibit threshold.
A second window that is used to prevent recovering old information is a read inhibit window. The old information is probably substantially displaced from track center, so the head must be substantially off track to read it. Although an off track read cannot damage the information on the disk, it can recover this old information. Consequently, this can drive values for the read and write inhibit windows that are picked for the worst case head, while many heads could operate with wider windows.
Another important aspect of the servo process is the settle delay. The settle delay can be defined as the time from when the seek operation completes until the data transfer can reliably be executed. Factors that increase the settle delay are mechanical vibration induced by the seek operation, external vibrations induced by adjacent drives seeking, fans running, high temperature or low voltage to the drive, etc. Since such factors can""t be accounted for ahead of time, the settle delay is conservatively selected to ensure the reliability of the data transfer operation. For example, if the actuator vibrates sufficiently, then the head can write off track even with a conservative write inhibit window. This is because the position is sampled in time, and therefore the write inhibit window can only be set at specific instances. Consequently, larger write heads will require tighter write inhibit windows and/or longer settle delays to avoid the risk of adjacent track damage.
Based on the exponential growth of disk drive technology, there exists a two-fold desire to make the servo process perform as fast as possible while ensuring the reliability of the data being generated. As computer systems become bigger and bit densities and track densities are increased, the speed and accuracy of the servo system becomes even more important. What is also important is the hard error rate of the system since many applications differ in their requisite degree of precision. It is therefore desirable that servo systems become faster and more reliable in their ability to process data. It is also desirable that systems are designed based upon a pre-calculated constant hard error rate. The present invention addresses such a need.
A method and system in accordance with the present invention for minimizing the loss of data by a write head is disclosed. The method comprises providing a desired write head width based upon a hard error rate. The method further comprises adjusting at least one parameter of a servo control logic based upon a comparison of the width of the write head and the desired write head width, wherein the write head will not exceed the hard error rate when writing data.
Through the use of a system and method in accordance with the present invention a servo system can be designed based on a constant hard error rate. The write inhibit window and the settle delay for writing operations in a servo system will be optimized, based on the hard error rate, for each individual head in the disk drive configuration. This will increase the speed of the servo process without affecting the reliability of the data.